CONTRIBUTIONS TO SCIENCE. 



495 



The figures on Plate III. are copies of water-colour 

 sketches made by Professor Maxwell in the very early 

 periods of these investigations, and show the isochromatic 

 lines in unannealed glass in the case of a pentagon and a 



triangle. 



The remainder of the paper on elastic solids is taken up 

 with a discussion of a number of examples of great import- 

 ance to the engineer, such as the flexure of beams, the 

 torsion of cylinders, and the like. To these engineering 

 problems Maxwell again returned many years later, and his 

 paper " On Reciprocal Figures, Frames, and Diagrams of 

 Force," read before the Royal Society of Edinburgh, *7th 

 February 1870, received the award of the Keith Medal. 

 The Theory of Oersted's Piezometer, given in the paper of 

 1850, is, however, of great scientific value. In it Maxwell 

 points out that the behaviour of a vessel exposed to equal 

 pressures within and without depends only on its cubic 

 compressibility, while the relation between the cubic com- 

 pressibility and the rigidity in solids, so far from being con- 



